Invited Session: Statistics in Information Technology

Organizer: Gordon Lyon, NIST

Session Chair: Jon Kettenring, Bellcore

Network Traffic Self-Similarity & the World Wide Web

Mark E. CrovellaDept. of Computer Science, Boston Univ.

Recently, the fractal-like property ofem self-similarity
has been found
in time-series distributions for
wide and local-area computer network traffic measurements.
Since an entity with self-similarity appears
unchanged despite a wide range of viewing scales,
this discovery has serious implications for the
performance modeling and evaluation of
networks and network protocols.
The mechanisms that give rise
to self-similar network traffic are illustrated nicely
by a common yet representative example:
traffic on the World Wide Web (WWW).
I shall discuss empirically obtained distributions both from our
own traces and from other data collected independently at over thirty WWW sites.
These records of actual user executions of NCSA Mosaic give strong
evidence that WWW traffic is self-similar. The fractal-like nature
originates in underlying
distributions of WWW document sizes, effects of caching and user
preference in file transfer, the effect of user "think time" and
superimposed transfers in a local-area network.

On the NASA Space Shuttle
software project, we learned that remaining
failures, total failures, test time required to attain a given fraction of
remaining failures, and time to next failure are useful reliability metrics
for: 1) providing confidence that the software has achieved reliability
goals; 2) rationalizing how long to test a piece of software; and 3)
analyzing the risk of not achieving remaining failure and time to next
failure goals. Having predictions of the extent that the software is not
fault free (remaining failures) and whether it is likely to survive a
mission (time to next failure) provide criteria for assessing the risk of
deploying the software. Furthermore, fraction of remaining failures can
be used as both a program quality goal in predicting test time
requirements and, conversely, as an indicator of program quality as a
function of test time expended.